Line-at-a-time addressing is a technique well known in the art (e.g. passive matrix OLED displays), and is based upon selecting one line of the display at a time, consecutively during the frame period, and while each line is selected addressing the pixels in this line. As a consequence, each pixel can only be addressed for a fraction of the frame period (i.e. the frame period divided by number of lines). Therefore, line-at-a-time addressing requires quite a large maximum intensity per pixel, in order to obtain the desired light output.
A conventional foil display is shown in FIG. 1, and comprises a light guide 1 in the form of an edge lit glass plate and a non-lit front plate 2, with a scattering foil 3 clamped in between. On both plates there are respective sets of parallel electrodes 4, 5 which are arranged perpendicularly with respect to each other. By application of voltages to appropriate electrodes on the light guide, the front plate and the foil, it is possible to generate two electrostatic fields with the field vectors directed towards the light guide and the front plate respectively. The balancing of these two electrostatic forces in combination with the elastic force of the foil is used to attract the foil to either the light guide or the front plate. Typically, the foil can be attracted towards the light guide using a column electrode and towards the front plate using a row electrode. When the foil is brought into contact with the light guide, light is extracted and emitted through the front plate. If preferred, the front plate can include a color filter and/or a black matrix.
In order to minimize absorption, the light guide is made relatively thick, so as to reduce the number of reflections by the light guide surfaces. This means that the amount of light that can be extracted from the light guide per unit length, which is proportional to the number of times each light ray is reflected, is relatively small. Therefore, line-at-a-time addressing is not possible. Simply put, with light rays traveling in the column direction, each light ray does not hit all the pixels in a column.
Instead, a sub-frame addressing scheme is used, making use of the bi-stability of the foil. This is described in WO 00/38163, with several positive effects.
In practice, however, the control of the bi-stable switching is difficult, as non-homogeneous switching curves can cause certain pixels to remain ON or OFF. It also requires a large number of pixel switching events during addressing. Additionally, sub-frame addressing requires complex and expensive electronics.